Local protesters in a dozen small boats demonstrated against the controversial aircraft's arrival, chanting "We don't want the dangerous Osprey!" and "Osprey, go back to America".

The demonstration against the unloading of the 12 aircraft would continue throughout the day, protest organiser Kiyoshi Oka told AFP by telephone.

Although local governments in Japan have no legal grounds to reject the US deployment plan, strong local resentment, in particular in Okinawa where the aircraft will be based, could further erode public support for the government of Prime Minister Yoshihiko Noda.

The US military plans to fully deploy Osprey aircraft to Okinawa in October, while the governor of the sub-tropical Japanese island chain has denounced the US plan because of safety concerns.

Following checkups at Iwakuni, the aircraft is destined for the Marine Corps airbase of Futenma in Okinawa, which has been at the centre of a long-running stand-off as it sits in a developing urban area.

A separate rally was held outside the Futenma base Monday with protesters holding banners that said "We are opposed to deployment", Jiji Press reported.

A huge US military presence in Okinawa, accounting for around half of the 47,000 troops Washington has in Japan, has angered islanders there.

Concerns over the Osprey came after the two countries clinched a deal earlier this year under which the United States will shift 9,000 Marines out of Japan in a step designed to ease friction with Tokyo over the US military footprint.

The Osprey is a hybrid aircraft with rotors that allow it to take off like a helicopter and engines that can tilt forward, enabling it to fly like an airplane at greater speed than a chopper.

The aircraft was plagued with problems in its early years in the 1990s, but US officials say the technical glitches have been cleared up and the US Marine Corps says it has proven invaluable.

A US Air Force CV-22 Osprey crashed in Florida in June, injuring all five crew members. US officials said the accident was not due to mechanical problem.

In April, an MV-22 Osprey -- the variant that arrived in Japan -- crashed in Morocco, killing two Marines.

The mayor of Iwakuni as well as three members of Noda's Democratic Party of Japan have also voiced their opposition to the Osprey deployment.

The deployment of the Osprey "is a vital component in fulfilling the United States' commitment to provide for the defence of Japan and to help maintain peace and security in the Asia-Pacific region", the US embassy in Japan said in a statement issued Monday.

"The United States and Japan have agreed that US forces in Japan will refrain from any flight operations of the MV-22 in Japan until the results of investigations into recent incidents in Morocco and Florida are presented," it said.

The probe results are expected to be delivered to Tokyo officials by August, the embassy said.

Chief Cabinet Secretary Osamu Fujimura, the government's top spokesman, told reporters Monday that "we will make utmost efforts in obtaining understanding" from local residents on the Osprey deployment by providing information on US probes into the latest accidents.

Japanese Defence Minister Satoshi Morimoto said Japan will send its own investigation team to Washington.